Distemper in the dog an measles in man, both paramyxovirus infections, may result in demyelinating encephalopathies known as post distemper encephalitis and subacute sclerosing panencephalitis (SSPE) respectively. Evidence is accumulating that multiple sclerosis (MS) may also be a virus-induced demyelinating encephalitis. The process of demyelination, characterized by remissions and relapses probably depends upon a persistent infection and an interplay between host and agent whereby immunological factors probably play a key role. Research in this field was greatly impeded because of the lack of a suitable experimental model in which the factors responsible for the disease could be individually tested and the course of the disease accurately followed. Such a model has now been developed through the use of a distemper virus isolate capable of including a demyelinating encephalomyelitis in gnotobiotic pups after an incubation time of 5-6 weeks. Experiments have been designed to permit careful dissection of individual mechanisms and their importance for the production of demyelinating encephalitis.